The present invention relates generally to an antenna system for a vehicle radio and more particularly to an active antenna system where antenna elements are printed on a window glass of the vehicle. Most vehicle antenna systems include a mast antenna that provides acceptable AM and FM reception. Such antennas have certain disadvantages, including wind noise, increased drag on the vehicle, corrosion of the antenna, and risk of vandalism.
Printed on car window glass antennas do not have such disadvantages. On the other hand, it is known (K. Fujimoto and J. R. James, Mobile Antenna Systems Handbook, Artech House 1994, pages 297-311) that the gain of passive screen (i.e. printed glass) antennas is between xe2x88x926 to xe2x88x9210 dB compared to a standard mast or whip antennas in the AM and FM frequency range. This means that the passive antenna structure does not meet the performance of a whip antenna. An active screen antenna (i.e. printed on glass antenna plus amplifier) can improve antenna performance in this situation. An increase in signal to noise ratio when using an active screen on glass antenna system compared to the whip antenna can be calculated using the following formula:
xcex3=Gprint/Gwhip*Frec/(Famp+(Frecxe2x88x921)/Gamp)xe2x80x83xe2x80x83(Eq. 1)
where, Gprint=gain of the printed on glass antenna; Gwhip=gain of the whip antenna; Frec=noise figure of the receiver; Famp=noise figure of the amplifier; and Gamps=gain of the amplifier. Procedures used to obtain values of the foregoing variables are well known in the industry, and therefore need not be described in detail.
With attention to Eq. 1, as an example, it is assumed the gain of a whip antenna (Gwhip) is ten times more than the gain of a printed on glass antenna (Gprint) and noise figure of the receiver (Frec) is ≈10. If the amplifier gain (Gamp) is equal to 10, and the noise figure of the amplifier (Famp) is ≈3, the value of xcex3 calculated by equation 1 is to about 1 (more specifically it is 0.83). This means that the signal to noise ratio using either a whip antenna or a printed on glass active antenna system is approximately the same. Therefore a satisfactory signal to noise ratio can be obtained using a glass antenna with an amplifier.
Printed on glass active antenna system are described in many different publications, for example U.S. Pat. Nos. 4,163,195; 4,757,277; 4,791,426; 4,260,989. In all these prior art antenna systems the antenna amplifier does not have an overload protection circuit. Some of them do however have excellent linear dynamic range. As an example, an amplifier described in the Mobile Antenna Systems Handbook, page 305, with an output amplifier signal equal to xe2x88x927 dBm (50 Ohm input impedance) generates an intermodulation product, exceeding the noise level by only 2 dB. On the other hand it is known that the radio frequency (RF) electromagnetic signal in some areas (for example, cities such as New York, Detroit, etc.) can reach extremely high levels of up to 10 dBm or more. Such extremely high RF signal levels generate a high intermodulation distortion signal in the RF amplifier.
A third order intermodulation signal level P3 can be expressed as follows.
P3=3POxe2x88x922IP3xe2x80x83xe2x80x83(Eq. 2)
Here PO is the output amplifier power, and IP3 is the third order interception power level. It is seen from this expression that P3 can be very high. For example, an output amplifier signal of 20 dBm causes an intermodulation distortion level P3 of xe2x88x9210 dBm if the third order interception power level of the amplifier IP3=35 dBm. When there are two high level received signals with frequencies F1 and F2, a third order signal having a frequency F3=2F2xe2x88x92F1 or F3=2F1xe2x88x92F2 is produced. If the two frequencies F1 and F2 are present at relatively near frequencies, a signal generated by the intermodulation has approximately the same frequency as that of another broadcasting station. Therefore intermodulation interference occurs and the signal of the desired station with frequency F3 can be mixed and distorted.
An overload protection circuit such as described in U.S. Pat. No. 5,230,096 automatically estimates an output antenna signal level in an operating frequency range and sufficiently decreases the output intermodulation product when the output antenna signal is out of the linear amplifier dynamic range. This overload protection circuit bypasses the RF amplifier when the signal applied to the amplifier is out of linear dynamic range of the amplifier. However, when the bypassing process has been started, the sensitivity (i.e. the signal to noise ratio) of the active antenna system drops off. It is desirable therefore to smooth out this sudden degradation of signal sensitivity by using additional automatic gain control on the signal.
An object of the present invention is to provide broadcast signal reception using an active antenna system without intermodulation distortion while improving sensitivity when the overload protection circuit is in operation.
According to the present invention the overload protection circuit is installed between the antenna and the amplifier. This circuit comprises a directional coupler, a detector for detecting an envelope level of an output antenna signal, an operational amplifier for amplification of a DC signal, a variable value shunt impedance controlled by the output signal of the operational amplifier, a comparator, an electronic switch and matching circuits. When the received antenna signal is at a typical operating level of the amplifier, the overload protection circuit connects the output of an antenna to the RF amplifier. When the received signal is out of the linear dynamic range of the amplifier, the overload protection circuit provides linearity of the signal that is applied to the radio.
This process is comprised of two steps. In the first step, the overload protection circuit automatically decreases the signal applied to an amplifier so the amplifier output signal does not change. In the second step, when the output antenna signal is very high, electronic switching of the overload protection circuit automatically provides bypassing of the amplifier and connects the antenna directly to the car radio without an amplifier.